A surface-mounting type coil component is now widely used in consumer or industrial electronic equipment. Particularly, in small mobile equipment, there has occurred, along with its enhancement of functionality, a need to obtain a plurality of voltages from a single power supply in order to drive various devices provided therein. Such a coil component for power supply use is demanded to be small/thin, excellent in insulating performance and electrical reliability, and to be manufactured at low cost.
As a structure of a coil component that meets the above requirement, a planar coil structure based on a printed circuit board technology is known. The coil component of such a type has a structure in which planar coil patterns are formed respectively on both top and back surfaces of a printed circuit board and the printed circuit board is sandwiched between, e.g., EE type or EI type of sintered ferrite cores. With this configuration, a closed magnetic path is formed around the planar coil patterns.
The coil component for power supply use is required not to exhibit a decrease in inductance thereof due to magnetic saturation even when a certain high direct bias current is applied thereto. To meet the above requirement, a coil component described in Patent Document 1 has first and second magnetic layers covering upper and lower surfaces of an insulating substrate on each of which a planar spiral conductor is formed, and these two magnetic layers each have a gap in a thickness direction at an outer edge area of the coil pattern. This can suppress magnetic saturation in a magnetic circuit to increase an inductance of the magnetic circuit.
Patent Document 2 discloses a coil component having a structure in which an air-core coil is embedded in a packaging resin to be integrated therewith. This coil component includes a resin containing metal magnetic powder. In particular, by using a compound material in which two or more types of amorphous metal magnetic powder having different average particle diameters and an insulating binder are mixed with each other, it is possible to obtain high density, high magnetic permeability, and low core loss even under low pressure molding conditions.
In a field of commercial or industrial electronic equipment, the surface-mounting type coil component has come to be used frequently as a power supply inductor. This is because the surface-mounting type coil component is small/thin, excellent in insulating performance, and capable of being manufactured at low cost.
A planar coil structure using a printed circuit board technology is known as one of a specific structure of the surface-mounting type coil component. The following briefly describes the planar coil structure in terms of a manufacturing process thereof. First, a seed layer (base film) having a planar spiral conductor shape is formed on a printed circuit board. Then, the resultant circuit board is immersed in plating solution, and DC current (hereinafter, referred to as “plating current”) is applied to the seed layer to cause metal ions in the plating solution to be electrodeposited onto the seed layer. As a result, a planar spiral conductor is formed and, thereafter, an insulating resin layer covering the formed planar spiral conductor and a metal-magnetic-powder-containing resin layer serving as both of a protective layer and a magnetic path are sequentially formed, whereby manufacturing of the coil component is completed. This structure allows high dimensional and positional accuracy to be maintained, as well as, a reduction in size and thickness. Patent Document 1 discloses a planar coil element having such a planar coil structure.